If you have the time and resources, I recommend a pilot where you can check the accessibility yourself, as well as other business requirements. Many Software-as-a-Service offerings are happy to give you free trial periods once they realize you are a serious customer. We did paid proofs of concept for our enterprise web content management system, since it was more elaborate to get set up as we would be using it and because it allowed us more realistic tests. We selected the top two products from a more traditional procurement screening process and paid for installation and configuration of test environments.

One thing we learned from it is that while VPATs are invaluable (we weeded out multiple bidders whose VPATs were just "does not support") their reliability depends on the vendor's knowledge of accessibility. The VPAT from a knowledgeable vendor may look worse than from a vendor with limited accessibility expertise: it looks like they have a lot of problems because they report on them in detail while the other cheerfully states "Supports" in every box. Conducting testing on both ourselves gave us an apples-to-apples comparison to work with. It also allowed us to see the context of reported errors so we could determine if they presented barriers to task completion.

While paying for a trial is more costly than free, it paid for itself by actually demonstrating which product best fit our needs. If we hadn't done it, we would have made the costly mistake of acquiring the wrong product. And the added bonus was that we had figured out most of the complicated configurations, so that went quickly in setting up the final product.